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Benefit Plans Components of Benefit Plans (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
plans
Jun. 30, 2019
USD ($)
Retirement Plan Disclosure        
Number of unfunded post-retirement health care plans | plans     2  
Pension Plan        
Retirement Plan Disclosure        
Service cost $ 14 $ 11 $ 41 $ 33
Interest cost 104 125 312 374
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (122) 119 (366) 358
Amortization of prior service cost (25) (25) (73) (74)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 109 (84) 327 (252)
Net periodic benefit cost as acutarially determined 80 76 241 227
Amount capitalized due to actions of regulator 3 0 11 1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 77 76 230 228
Other Postretirement Benefit Plan        
Retirement Plan Disclosure        
Service cost 4 2 12 8
Interest cost 4 5 12 14
Defined Benefit Plan, Expected Return (Loss) on Plan Assets 0   0 0
Amortization of prior service cost (6) (6) (18) (18)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 2 (1) 7 (3)
Net periodic benefit cost as acutarially determined 4 2 13 7
Amount capitalized due to actions of regulator 0   0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ 4 $ 2 $ 13 $ 7